Merge branch 't/sched_improvements'

Merge branch 't/sched_improvements'

Cooking for two months. This merge required to also patch gui.c due
to semantic conflics against the changes introduced by the gui_sched
branch which was merged to master in commit d15d8509 two weeks ago.
Also a small fix for server.c is needed to squelch a compiler warning
since the global "now" variable has become a const pointer.

* t/sched_improvements: (36 commits)
  audiod: Fix use after free on exit.
  sched: Mark global now pointer as const.
  sched: Directly pass context pointer to pre/post_select().
  sched: kill task->dead.
  sched: Do not shadow task_info in struct task.
  sched: Dont use fixed-size buffer for task names.
  sched: Rename task->error to tast->status.
  sched: Rename task->status to task->name.
  sched: Make struct task private to sched.c.
  sched: Introduce task_status().
  sched: Remove ->owned_by_sched.
  sched: Remove register_task().
  task_register() conversion: grab client task
  task_register() conversion: audiod status task
  task_register() conversion: audiod command task
  task_register() conversion: client task
  task_register() conversion: client supervisor task
  task_register() conversion: client exec task
  task_register() conversion: afs command task
  task_register() conversion: vss task
  ...